In recent years, elements have been more highly integrated in an integrated circuit. A process rule as a process condition for manufacturing an integrated circuit on a semiconductor wafer is ordinarily defined by a Minimum Feature Size in a line width or an interval of gate wires. When the process rule is halved, since theoretically, four times transistors or wires can be arranged in the same size of an area, the same number of transistors need one-fourth of the area. As a result, since not only the number of dies that can be manufactured from one semiconductor wafer is quadrupled but also a yield is ordinarily improved, more dies can be manufactured. The most advanced Minimum Feature Size for manufacturing a high density integrated circuit as of 2013 reaches 22 nm. Such a process rule on the order of sub-micrometer (1 μm or less) requires high flatness of a semiconductor wafer, and thus a surface shape (a change in a height of a surface) of a semiconductor wafer is not negligible. A shape measurement device is therefore demanded that measures a surface shape of a semiconductor wafer with high accuracy, for example, on the order of sub-nanometer (1 nm or less).
Here, a thin plate-shaped measurement object such as a semiconductor wafer may vibrate due to, for example, slight air pressure or vibration of another device, etc. Vibration caused in a measurement object might have an amplitude not negligible for the shape measurement with high accuracy. Therefore, the shape measurement with high accuracy needs a countermeasure against vibration of a measurement object. A shape measurement device having such a countermeasure against vibration is disclosed, for example, in Patent Literature 1 and Patent Literature 2.
The profile measuring apparatus disclosed in Patent Literature 1 is a profile measuring apparatus used for scanning front and back surfaces of a workpiece and measuring a thickness distribution of the workpiece in a non-contact manner, the profile measuring apparatus including first optical branching means for branching source light that is emitted from a predetermined light source into two pieces of light; optical guiding means for guiding the pieces of light branched by the first optical branching means in directions toward measurement portions on the front and back surfaces of the workpiece, the measurement portions facing each other; second optical branching means for further branching each of the pieces of light branched from the source light guided in the directions toward the measurement portions on each of front and back of the workpiece into two pieces of light; optical modulating means for modulating a frequency or frequencies of one or both of the pieces of light branched by the second optical branching means at each of the front and back of the workpiece, and generating two pieces of measurement light with different frequencies; two heterodyne interferometers that irradiate the measurement portion with one of the pieces of measurement light, and causes object light, which is the one of the pieces of measurement light reflected by the measurement portion, to interfere with reference light, which is the other of the pieces of measurement light, at each of the front and back of the workpiece; third optical branching means for branching each of the two pieces of measurement light into two pieces of light including main light, which is input to the heterodyne interferometer, and sub-light, which is other than the main light, at each of the front and back of the workpiece;
sub-light interfering means for causing the two pieces of sub-light branched by the third optical branching means to interfere with each other, at each of the front and back of the workpiece; measurement optical system holding means for integrally holding a measurement optical system including the second optical branching means, the optical modulating means, the heterodyne interferometers, the third optical branching means, and the sub-light interfering means, at each of the front and back of the workpiece; measurement light intensity detecting means for receiving pieces of interfering light obtained by the two heterodyne interferometers and outputting intensity signals of the pieces of interfering light; reference light intensity detecting means for receiving interfering light obtained by the sub-light interfering means and outputting an intensity signal of the interfering light, at each of the front and back of the workpiece; and phase information detecting means for detecting phases of two beat signals including an output signal of the measurement light intensity detecting means and an output signal of the reference light intensity detecting means, and detecting a phase difference between the two beat signals, at each of the front and back of the workpiece. According to the recitation of Patent Literature 1, in the profile measuring apparatus having such a configuration as disclosed in Patent Literature 1, a measurement value of a thickness of the workpiece will be a measurement value obtained by cancelling a component of a displacement amount caused by vibration of the workpiece on both the front and back of the workpiece. Accordingly, the profile measuring apparatus is allowed to measure a thickness of the workpiece without being affected by vibration of the workpiece.
The measurement apparatus disclosed in Patent Literature 2 includes a mount unit configured to mount an object; a probe configured to move with respect to the object so as to measure a shape of the object; an interferometer configured to measure a position of the probe based on reflected light obtained by irradiating a reference mirror with light; and a calculator configured to calculate a shape of the object using a measurement value relating to the shape of the object that is obtained by moving the probe, and a relative displacement amount between the object and the reference mirror that is obtained based on a signal from a sensor for the object and the reference mirror. According to one mode, the sensor is a displacement amount sensor that detects the relative displacement between the object and the reference mirror, and the calculator corrects the measurement value using the relative displacement amount that is detected by the displacement sensor so as to calculate the shape of the object. According to another mode, the sensor is an acceleration sensor that detects a relative acceleration between the object and the reference mirror, and the calculator performs a second order integration of the relative acceleration so as to calculate the relative displacement amount between the object and the reference mirror, and corrects the measurement value using the relative displacement amount so as to calculate the shape of the object. According to the recitation in Patent Literature 2, a measurement apparatus having such a configuration enables measurement of a shape of an object with high accuracy even when a relative displacement is generated between a reference mirror and the object.
In a shape measurement device, however, not only a measurement object vibrates but also a measurement unit (sensor unit) for measuring a shape of the measurement object itself might vibrate. For example, in a case of Patent Literature 1, two heterodyne interferometers themselves might vibrate. Additionally, in a case of Patent Literature 2, for example, the probe itself might vibrate. Since such a vibration of the measurement unit itself also causes a change of a distance between a measurement unit and a measurement object, a countermeasure against vibration of a measurement unit itself is also demanded in the above shape measurement with high accuracy.
As described above, since the profile measuring apparatus disclosed in Patent Literature 1 cancels a component of a displacement amount caused by vibration of a workpiece on both the front and back of the workpiece, the apparatus is capable of coping with vibration of the workpiece itself but not with vibration of the measurement unit itself. Additionally, the measurement apparatus disclosed in Patent Literature 2 is capable of copying with relative displacement between a reference mirror and a workpiece but not with vibration of the measurement unit itself as described above.